Counting elements inside an array in mongodb

Question

I want to count the number of elements in an array using python and pymongo. Here is the data.

{
"_id": 5,
"type": "Student",
"Applicates": [
    {
        "appId": 100,
        "School": "dfgdfgd",
        "Name": "tony",
        "URL": "www.url.com",
        "Time": "5/5/5-6/6/6",
        "Research": "dfgdfg",
        "Budge": 5000,
        "citizenship": "us",
        "Major": "csc",
        "preAwards": "None",
        "Advisor": "dfgdfg",
        "Evaluators": [
            {
                "abstractScore": 10,
                "goalsObjectivesScore": 20,
                "evalNum": 1
            },
            {
                "abstractScore": 30,
                "goalsObjectivesScore": 40,
                "evalNum": 2
            },
            {
                "abstractScore": 50,
                "goalsObjectivesScore": 60,
                "evalNum": 3
            }
        ]
    },
    {
        "appId": 101,
        "School": "dvdu",
        "Name": "jessy",
        "URL": "www.url.com",
        "Time": "4/4/4-6/6/6",
        "Research": "dfgdfg",
        "Budge": 7500,
        "citizenship": "us",
        "Major": "dfgdfg",
        "preAwards": "dfgfd",
        "Advisor": "dfgdfg",
        "Evaluators": [
            {
                "abstractScore": 70,
                "goalsObjectivesScore": 80,
                "evalNum": 1
            },
            {
                "abstractScore": 90,
                "goalsObjectivesScore": 100,
                "evalNum": 2
            }
        ]
    }
]}

So I want to get the size of the Evaluators array. {"appId" : 100} would give 3 and {"appId" : 101} would give 2. I have been playing around with $size but cant seem to get it.

wdberkeley · Accepted Answer · 2015-02-13 15:48:22Z

1

Queries return documents. No query will return the size of the Evaluators array in the array element with "appId" : 100`. But the following awkwardly formatted expression will so what you want:

len(coll.find_one(
    { "Applicates.appId" : 100 }, 
    { "Applicates.$.Evaluators" : 1 }
)["Applicates"][0]["Evaluators"])

where coll is the Collection object.

answered Feb 13, 2015 at 15:48

wdberkeley

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1 Comment

Antespo Over a year ago

Thanks this works pretty much how I wanted. I was trying to do an aggregation with $size which also seems to do exactly what I wanted but I couldn't seem to get the correct <expression>.

Nejc Rodošek · Accepted Answer · 2015-02-13 01:15:04Z

0

With this syntax { $size: <expression> } you can count number of items in an array. See here for more > $size

answered Feb 13, 2015 at 1:15

Nejc Rodošek

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1 Comment

Antespo Over a year ago

Ya I was on the page but since I just started using mongdb a week ago my understand is still pretty low. I'm confused on the expression needed to get to my array within an array

Taylor Kolasinski · Accepted Answer · 2015-02-13 15:23:44Z

0

One approach would be to loop through your array, and then on each iteration use len() on that dict's Evaluators property.

for obj in Applicates:
    count = len(obj['Evaluators'])

edited Feb 13, 2015 at 15:23

answered Feb 13, 2015 at 1:13

Taylor Kolasinski

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2 Comments

Antespo Over a year ago

This does work but how would I specify the array elements for each individual application?

Taylor Kolasinski Over a year ago

What do you mean exactly? @Antespo

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